There are many technologies and scientific areas where it is desirable to create and control strong magnetic fields. Strong magnetic fields have many applications, for example, in MHD (magnetohydrodynamic) generation of energy and in nuclear fusion processes, which require magnetic confinement of a plasma, in mass spectroscopy in which a narrow beam of ions is deflected by a strong magnetic field, in NMR (nuclear magnetic resonance) spectrometers, and in concentration of microwaves for communication or for cancer treatment. Strong magnetic fluxes are generally obtained by means of superconductive electromagnets which, by using inductance coils made of superconductor materials such as niobium, titanium, or columbium, or alloys thereof cooled to cryogenic temperatures are capable of producing flux densities of 20,000 to 140,000 gauss.
One problem associated with any magnets or electromagnets, even those capable of producing only relatively weak magnetic fields as well as those capable of producing very high flux intensities, is that of magnetic flux leakage and dispersion to the ambient. The higher the magnetic flux created, the greater of course the problems presented by confinement of the magnetic field and controlled concentration of the magnetic lines of force, so that the high intensity magnetic flux is concentrated in a desirable region of the magnetic field such as, for example, along the centerline of the pole piece gap, or along a cylindrical or frusto-conical periphery concentric with the longitudinal axis of the magnetic gap, rather than obtaining a magnetic field having a conventional barrel shape with maximum deflection of the field being at mid-distance between the pole piece faces.
Attempts have been made in the past to provide magnetic concentrators or shields, sometimes associated with the pole pieces of a magnet or electromagnet, to reduce flux leakage, or to provide a better control of the shape of the magnetic flux between the opposite pole pieces of the magnet or electromagnet. An example of such shield structure can be found in U.S. Pat. No. 3,781,736 which discloses providing the pole pieces of a magnet with sleeves or plates engaged with the peripheral surface of the pole pieces, and in U.S. Pat. No. 3,417,356 disclosing hollow magnetic elements placed around the opposed pole pieces of a magnet to effectuate a correction of the field gradient at the edge of the magnetic field in the air gap between the pole pieces.